The present invention relates to apparatus for mechanically stirring a quench-cooled subfluidized particulate bed of polymerized monomer during continuous vapor phase polymerization in horizontally disposed cylindrical polymerization reactors. More particularly, the invention is an improved apparatus for stirring polymer particles in reactive gas-filled polymerization reactors incorporating contiguous paddle stations on a coaxial drive shaft within the reactor with a plurality of sub-stations having lengths along the shaft.
U.S. Pat. No. 3,639,377 (Trieschmann et al.) describes polymerization of propylene which is carried out in the gas phase. In order that heat of polymerization should be effectively removed, excess monomeric propylene is introduced in liquid or partly liquefied form into the bottom of a vertically disposed cylindrical reaction zone. During polymerization, the fraction of unpolymerized propylene evaporates while absorbing the heat of polymerization. Evaporated propylene is removed from the reaction zone and condensed again outside the reaction zone. While stating that removal of heat according this system of internal cooling also causes intense mixing of the solid polymer powder with the gas phase, Trieschmann et al. state that it is particularly advantageous to use a reactor having a spiral stirrer. Referring to FIGS. 1 and 3 of U.S. Pat. No. 3,639,377, the stirrer appears to be illustrated as having a xe2x80x9cUxe2x80x9d shape and rotates on a vertical shaft through the bottom of vertical reactor 6. Catalyst is pumped into the reactor through the top and polymer is discharged from the reactor by means of an external cyclone. Advantageous use of high activity catalysts in a continuous polymerization process requires, however, at least a plurality of the single-stage back-mixing reactor type described in Trieschmann et al.
U.S. Pat. No. 3,944,534 (Sennari et al.) describes gas-phase polymerization of an xcex1-olefin which is carried out in a reaction bed formed by circulation of particulate olefin polymer, caused principally by mechanical agitation to undergo circulation in the up-and-down directions within a substantially vertical-cylinder type reactor. The single-stage back-mixing reactor type described in Sennari et al. likewise is not suitable for use in a continuous polymerization process with high activity catalysts, because age of catalyst carried out of the reactor is substantially the same as the age of catalyst in a back-mixing reactor.
Vapor-phase polymerization of a polymerizable monomer or mixture thereof to produce normally solid polymer substances using a horizontal polymerization reactor containing a subfluidized particulate bed of polymerized monomer has been described in a number of patents including: U.S. Pat. No. 3,957,448 (Shepard et al.), U.S. Pat. No. 3,965,083 (Jezl et al.), U.S. Pat. No. 3,971,768 (Peters et al.), and U.S. Pat. No. 4,627,735 (Rose et al.), the disclosures of which are specifically incorporated herein in their entirety by reference. These U.S. Patents, assigned to the assignee of the present invention, describe polymerization processes and apparatus in which polymer is formed from gaseous monomer in horizontal stirredbed vessels.
In a single reactor, polymerization of monomer or mixture thereof from the vapor state is carried out by an essentially isobaric process typically using a high yield catalyst and cocatalyst. Typically, in operation of such processes and apparatus, particles of polymer are formed around solid catalyst particles.
The horizontally disposed reactor vessel has recycle gas, such as propylene, introduced into the bottom thereof. Typically, quench liquid, such as liquid propylene, is injected into the reactor from the top of the reactor.
Gases and vapors within the reactor vessel are free to circulate and mix together throughout the vapor space. For continuous production of some polymers, particularly copolymers, where it may be necessary to have different gas compositions at subsequent stages of polymerization, a series of two or more reactors is required.
Paddle wheels or other types of stirring vanes inside the vessel sweep through the bed of polymer particles and stir the contents of the vessel. The various types of stirring vanes include staggered paddles, inclined paddles, spiral vanes, or vanes provided with a scraper for scraping the internal wall of the reactor vessel.
Near one end (front end disposed opposite to a take-off end) of the horizontal vessel, a solid transition metal-containing catalyst component is injected at least one point into the top of the vessel, and an aluminum alkyl cocatalyst plus modifiers are injected at an adjacent the point at the top of the vessel.
Solid particles of polymerized monomer are created in the vessel and are withdrawn from the take-off end thereof. Particles of polymerized monomer build up in the stirred reactor and traverse the length of the reactor essentially because of polymerization in the bed and not by the agitator. Advantageously, this condition is ensured by the design of the agitator such as to provide for agitation, but not for significant backward or forward movement of the particles. Since a stirred bed is not in a fluidized condition, back-mixing of the particles of polymerized monomer in the horizontally disposed reactor vessel is limited. In contrast, solid particles in a fluidized bed are very well mixed. Even at commercially useful ratios of length to diameter, horizontal stirred-bed reactor systems can readily achieve a degree of mixing of solids equivalent to two, three, or more theoretical back-mix reactors. Thus, horizontal stirred-bed reactor systems are particularly advantageous, as compared fluidized-bed reactors, for direct production of polymers in a particulate no form.
It is desirable to create polymer particles as quickly as possible, and for this purpose a number of different high activity catalyst systems have been developed.
Use of solid, transition metal-based, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halidebased catalyst components. Such catalyst components commonly are referred to as xe2x80x9csupported.xe2x80x9d
As is well known in the art, particulate polymers and copolymers may be sticky, i.e., tend to agglomerate, due to their chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers also are referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor, however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize.
Although polymers that are sticky can be produced in non-gas phase processes, there are certain difficulties associated with the production of such products in, for example, slurry or bulk monomer polymerization processes. In such processes, the diluent or solvent is present in the resins exiting the reaction system at a high concentration leading to severe resin purging problems particularly if the material in question is a low molecular weight resin or a very low crystallinity resin. Environmental considerations are such that the dissolved monomers and diluent must be removed from the polymer prior to its exposure to air. Safety also dictates the removal of residual hydrocarbons so that closed containers containing the polymers will not exceed safe levels for volatiles in the gas head space over the resin. The safety and environmental concerns are accompanied by a definite economic factor in determining a preference for a quench-cooled, vapor-phase polymerization reactor containing a subfluidized particulate bed of polymerized monomer. The low number of moving parts and the relative lack of complexity in a basic subfluidized bed process enhances the operability of the process and typically results in lower costs of production. Low costs of production are due, in part, to low volumes of recycled process streams and a high unit throughput.
Horizontal stirred-bed reactor systems disclosed in Shepard et al., Jezl et al., Peters et al., and in U.S. Pat. No. 4,101,289 (""289), U.S. Pat. No. 4,129,701 (""701), U.S. Pat. No. 4,535,134 (de Lorenzo et al.), U.S. Pat. No. 4,627,735 (Rose et al.), U.S. Pat. No. 4,640,963 (Kreider et al.), U.S. Pat. No. 4,883,847 (Leung et al.), U.S. Pat. No. 4,921,919, (Lin et al.) and U.S. Pat. No. 5,504,166 (Buchelli et al.), the disclosures of which are specifically incorporated herein in their entirety by reference, largely or completely solve problems related to vapor phase, solution or slurry polymerization and reaps important economic benefits through savings in energy consumption, raw materials, and capital costs.
Although previously-known vapor-phase polymerization systems are entirely satisfactory for manufacture of many commercial polymers, a need still exists for improved mechanical stirring in a quench-cooled subfluidized particulate bed of polymerized monomer during continuous vapor phase polymerization. Desirably, the improved process produces fewer lumps and strings of resin. Such lumps and strings tend to hang-up or become trapped in transfer equipment and can even plug lines and valves. More desirably, the improved transfer apparatus increases the range in physical properties of polymers which can be manufactured at high rates of production without interruptions in operation. Especially welcome are improved methods and apparatus which more closely achieve continuous steady-state conditions throughout the vapor-phase process and thereby produce polymer products having more uniform physical properties.
One problem with known polymerization processes and apparatus using a vapor-phase polymerization system, is that lumps and strings of resin can form in a quench-cooled subfluidized particulate bed of polymerized monomer without reliable and accurate methods for mechanical stirring. Polymers formed from alkenes of 2 to 8 carbon atoms such as propylene or a mixture of propylene and other lower alkenes often have a tendency to agglomerate under operating conditions during polymerization. Such sticky polymers are difficult to maintain in granular or particulate forms during polymerization, particularly where high rates of production are desired. Further, it is advantageous to maintain a uniform temperature profile along the reactor. Agitator apparatus according to this invention advantageously are useful for stirring of polymer particles, particularly in subfluidized particulate beds of alpha-olefin polymers in high pressure, reactive gas-filled, continuous, vapor-phase polymerization reactors.
The invention is improved apparatus for stirring polymer particles in reactive gas-filled polymerization reactors incorporating contiguous paddle stations on a coaxial drive shaft within the reactor with a plurality of sub-stations having widths along the shaft of no more than 50 percent of a paddle station thereby subdivided. Sub-station paddles attached to the shaft at each sub-station sweeping through the particulate bed in combination with wider station paddles advantageously reduce lumping of polymer particles.
One aspect of this invention is apparatus for mechanically stirring polymer particles in a horizontally disposed cylindrical polymerization reactor containing subfluidized particulate polymer bed wherein at least a portion of the heat of polymerization is removed by evaporative cooling using a readily volatilizable quench liquid. The apparatus includes: (a) a drive shaft coaxially mounted for rotation about its longitudinal axis and predominantly within a cylindrical polymerization reactor; (b) one or more classes of substantially contiguous paddle stations disposed along the drive shaft within the reactor, each station having one or more station paddles attached to the shaft so as to sweep through a cylindrical zone within the reactor thereby defining station widths along the shaft; and (c) a plurality of sub-stations having widths along the shaft of no more than 50 percent of a paddle station thereby subdivided and one or more sub-station paddles attached to the shaft at each sub-station so as to sweep through a sub-section of the cylindrical zone within the reactor.
In some embodiments, a first class of stations is, advantageously, disposed within the reactor along a portion of the drive shaft in repeating groups of at least two stations and a second class of stations with sub-stations is disposed within the reactor along another portion of the drive shaft, preferably in repeating groups of at least one more than the number of stations in the repeating group of the first class, to further reduce lumping of polymer particles.
Apparatus for mechanically stirring polymer particles according to preferred aspects of the invention have at least one class of stations which is subdivided into multiple (up to six) substations, more preferably two to about four, and the number of stations in each repeating group of the class typically is one more than the number of substations in each station.
Apparatus for mechanically stirring polymer particles in reactive gas-filled vessels according to the invention is, advantageously, used in a vapor-phase polymerization system having at least one, preferably two or more serially operated, horizontally disposed cylindrical polymerization reactors, containing a subfluidized particulate polymer bed wherein at least a portion of the heat of polymerization is removed by evaporative cooling using a readily volatilizable quench liquid. Preferably, apparatus for mechanically stirring the polymer particles includes: (a) a drive shaft being coaxially mounted for rotation about its longitudinal axis and predominantly within a cylindrical polymerization reactor; (b) one or more classes of substantially contiguous paddle stations disposed along the drive shaft within the reactor, each station having one or more station paddles with a center line located along a radius perpendicular to the longitudinal axis of the drive shaft to which it is attached and a distal end positioned in close proximity to the inner surface of the reactor wall to define by rotation a cylindrical envelope having an outer diameter which typically is at least 0.985, and preferably 0.995, times the inner diameter of the cylindrical reactor; and (c) a plurality of sub-stations having widths along the shaft of no more than 50 percent of a paddle station thereby subdivided and one or more sub-station paddle attached to the shaft at each sub-station so as to sweep through a sub-section of the cylindrical zone within the reactor.
One aspect of this invention is a process for continuous vapor phase polymerization of a polymerizable monomer or mixture thereof to produce normally solid polymer in two or more horizontally disposed cylindrical polymerization reactors wherein at least a portion of the heat of polymerization is removed by evaporative cooling using a readily volatilizable quench liquid, each containing reactive gases, a quench-cooled subfluidized mechanically-stirred bed of particulate polymer. Typically, a stirring means includes a drive shaft being coaxially mounted for rotation about its longitudinal axis and a plurality of paddles each of which has a center line located along a radius perpendicular to the longitudinal axis of the drive shaft with a distal end positioned in close proximity to the inner surface of the reactor wall so as to sweep through a cylindrical zone within the reactor. The improved process comprises providing the stirring means with two or more classes of substantially contiguous paddle stations disposed along the drive shaft within at least one reactor; multiple sub-stations, in at least one paddle station class, each substation having a width along the shaft of no more than 50 percent of the total width of a paddle station; and one or more sub-station paddles attached to the shaft at each sub-station, so as to sweep through a sub-section of the cylindrical zone within the reactor to mechanically stir the polymer particles.
Another aspect of this invention is a process for continuous polymerization wherein a first class of stations is disposed within the reactor along xc2xc to xc2xe of the drive shaft in repeating groups of at least two stations and a second class of stations is disposed within the reactor along xc2xe to xc2xc of the drive shaft in repeating groups of at least more than the number of stations in the repeating group of the first class.
Yet another aspect of this invention is a process for continuous polymerization wherein each paddle station of the first class has three flat paddles an angular intervals of one hundred twenty degrees therebetween and the angular orientation of paddles on adjacent stations of the class is sixty degrees, and each paddle station of the second class has at least a pair of flat station paddles fixed to the drive shaft at angular interval of one hundred eighty degrees therebetween and is subdivided by three substations, the number of stations in each repeating group of the class is four and the angular orientation of paddles on adjacent stations of the class is equal to 45 degrees.
For a more complete understanding of the present invention, reference should be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention.